1. Field of the Invention
The present invention relates to an alignment apparatus, and more particularly to an alignment apparatus suitable for use in an alignment system of a projection type exposure apparatus used in manufacturing a semiconductor device, a liquid crystal display device or a thin film magnetic head.
2. Related Background Art
When a semiconductor device, a liquid crystal display device or a thin film magnetic head is to be manufactured by using photolithography technique, a projection type exposure apparatus transfers an image of a pattern of a photo-mask or a reticle (hereinafter collectively referred to as reticle) onto a photo-sensitive substrate through a projection optical system. Since the semiconductor device usually comprises a number of layer patterns, it is necessary in manufacturing it to transfer various reticle patterns onto the photosensitive substrate in a predetermined sequence. When a reticle circuit pattern is to be transferred onto the photo-sensitive substrate in which patterns have already been transferred to shot areas, it is necessary to precisely align the reticle pattern to be exposed to the respective shot areas of the photo-sensitive substrate.
A method of precise alignment is disclosed in U.S. Pat. No. 4,710,026 which is a two-beam interference system using two laser beams.
As shown in FIG. 6, in the two-beam interference system, a wafer mark WM which serves as an alignment mark having a phase type diffraction grating is formed in the vicinity of each shot area on a wafer 1 which serves as a photo-sensitive substrate. The wafer marks WM are formed at a pitch P along an X direction which is a position measurement direction. Two laser beams LB1 and LB2 are symmetrically irradiated to the wafer marks WM thorugh a projection optical system. The pitch P and incident angles of the laser beams LB1 and LB2 are selected such that a +1-order diffraction light LB11 from the wafer mark WM by the laser beam LB1 and a -1-order diffraction light LB21 from the wafer mark WM by the laser beam LB2 are parallel to each other and they are reflected normally to the wafer 1.
When the diffraction lights LB11 and LB21 are interfered by the alignment optical system through the projection optical system, a signal due to interference is produced. In a heterodyne system in which frequencies of the laser beams LB1 and LB2 are different, the signal due to the interference is one having a predetermined beat frequency, and the position of the wafer 1 along the X direction can be exactly detected by comparing a phase of that signal and a phase of a reference signal. Similarly, wafer marks are also formed along a Y direction which is orthogonal to the X direction, and the position along the Y direction is detected from those wafer marks. Reticle marks which serve as alignment marks and have amplitude type diffraction gratings are formed on the reticle and the position of the reticle is detected from those reticle marks. The alignment is attained by directly or indirectly setting the wafer marks and the reticle marks in a predetermined positional relationship.
FIGS. 7A and 7B show a prior art manufacturing process of a wafer mark WM which serves as an alignment mark. As shown in FIG. 7A, periodic recesses 1a are formed by etching on a surface of the wafer 1 along the X direction at a pitch P. Then, as shown in FIG. 7B, a metal film 2 is deposited by a sputtering method on the surface including the recesses 1a. Since the portions of the metal film 2 which corresponds to the recesses 1a are formed as recesses 2a, a phase type diffraction grating having a high reflection factor and a pitch P is formed. The pitch P is 6 .mu.m, for example, and a width D of the recess 2a of the metal film 2 along the pitch direction (measurement direction) is set in accordance with D/P=1/2, in the prior art.
However, as shown in FIG. 7B, in the prior art wafer mark WM, a step .DELTA.h is produced at the bottom of the recess 2a of the metal film 2 by the use of the sputtering apparatus, and the recess 2a is laterally asymmetric. When two laser beams are irradiated to such asymmetric wafer mark WM to detect the position, a detection error is included, because in the alignment by the two-beam interference system, only a specific period component of the wafer marks WM which serve as the alignment marks is extracted to determine a position of center of gravity and hence the detection error is included if asymmetric wafer marks WM are included.
Conventionally, the laser beams LB1 and LB2 irradiated to the wafer marks WM are both polarized in a circularly polarized state.
FIG. 8 illustrates a conventional optical system (isolator) for use in separating the laser beam directed to the wafer marks from the laser beam reflected from the wafer marks. Directed to a polarized beam splitter 51 are two laser beams LB1 and LB2 emitted from a combination prism that is not illustrated. The polarization of the incident laser beams LB1 and LB2 directed to the polarized beam splitter 51 is linear polarization in a direction parallel to the paper surface in FIG. 8, namely, the linear polarization of P-polarized light relative to a composition plane of the polarized beam splitter 51. Accordingly, the intact laser beams LB1 and LB2 transmit through the composition plane of the polarized beam splitter 51.
The P-polarized laser beams LB1 and LB2 transmitted through the polarized beam splitter 51 are circularly polarized by being transmitted through a quarter wavelength plate 52. They are reflected from a mirror 53 and irradiated to the wafer mark WM through, for example, a projection optical system. In this event, diffracted light beams (diffracted light beams LB11 and LB12 in FIG. 6) diffracted normally and upwardly from the wafer marks WM are circularly polarized in an inverse direction. The diffracted light beams that are circularly polarized in the inverse direction are directed to the quarter wavelength plate 52 through the projection optical system and the mirror 53. As a result, the so returned diffracted light is converted into S-polarized light on being transmitting through the quarter wavelength plate 52. The S-polarized diffracted light is totally reflected from the composition plane of the polarized beam splitter 51 and is directed to a photo-sensing device 59. In this conventional art, the loss of the amount of light is minimized with a combination of the polarized beam splitter 51 and the quarter wavelength plate 52.